Quantum melting of magnetic order in an Ising spin chain

Changes in the spin excitation spectrum in the Ising chain magnet CoNb2O6 as the applied transverse field quantum melts the spontaneous magnetic order, as observed by neutron scattering on OSIRIS [1]. a) Schematic phase diagram showing a caricature of the spin quasiparticles in the two phases. b,c) In the ordered phase neutrons scatter by creating two independently-propagating domain-wall quasiparticles leading to a broad continuum with dispersive boundaries. e) In contrast, in the high-field paramagnetic phase excitations are spin-flip quasiparticles with a sharp single-particle dispersion. View full-size image

The quantum phase transition paradigm for a 1D Ising chain placed in a transverse applied magnetic field has been realised experimentally for the first time, at ISIS.

Quantum systems have zero-point fluctuations even at zero temperature and when such fluctuations become strong they can drive transitions between distinct phases of matter. One of the most theoretically-studied paradigms for such a quantum phase transition is the one-dimensional (1D) chain of Ising spins placed in a transverse applied magnetic field. The field stimulates quantum tunnelling between the “up” and “down” spin orientations and above a critical field these quantum fluctuations become strong enough to “melt” the spontaneous magnetic order and stabilize a paramagnetic state, Fig 1a). We have recently realized this system experimentally for the first time by applying strong magnetic fields to the quasi-one-dimensional, low exchange Ising ferromagnet CoNb2O6.

We have used high-resolution single-crystal neutron scattering on IRIS and OSIRIS to probe how the spin excitations evolve in magnetic field and observed some of the key predictions for 1D Ising criticality, namely a dramatic change in the fundamental quantum character of the spin quasiparticles as the magnetic field quantum melts the spontaneous magnetic order. In the low field ferromagnetically-ordered phase (Fig 1b-c) a broad continuum is observed as characteristic of neutron scattering by a pair of domain-wall (soliton) quasiparticles, which interpolate between regions with magnetization “up” and “down”. In contrast, at high fields (Fig 1e) a single sharp mode dominates, as characteristic of a spin-flip quasiparticle of the high field paramagnetic state. Moreover, we also observed how the weak, but finite couplings between the 1D chains enrich the physics and lead to “confinement” of soliton quasiparticles into a fine structure of bound states. Just below the critical field we observed that the energies of the two lowest bound states approached the “golden” mean (1+ √5)/2=1.61..., a key prediction of the remarkable theory of confinement near quantum criticality, never explored experimentally before. Our results emphasize that quantum criticality can open up new avenues to experimentally realize otherwise inaccessible correlated quantum states.